Field of the Invention
The present invention relates to a zoom lens, and is particularly suitable as an image pickup optical system used in an image pickup device such as a video camera, a surveillance camera, a digital still camera, a broadcast camera, or a silver halide film camera.
Description of the Related Art
There is demand for an image pickup optical system in an image pickup device to nave a zoom lens with a small size, a wide angle of view, low aberration, and a high zoom ratio. As a zoom lens that easily achieves a high zoom ratio, a positive-dead zoom lens in which a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power are arranged in order from the object side to the image side is known.
Japanese Patent Application Laid-Open No. H8-82743 and U.S. Pat. No. 8,284,498 disclose a rear focus type four-unit zoom lens in which the second lens unit and the third lens unit are moved for variable magnification and the fourth lens unit is moved for correction of image plane changes associated with variable magnification and for focusing.
Such a positive-lead four-unit zoom lens relatively easily achieves a high zoom ratio with the whole system being reduced in size. However, in order to achieve high optical performance over the whole zoom range with a wide angle of view, it is important to appropriately set the refractive power of each lens unit, in particular the refractive power of the second lens unit, the lens structure of each lens unit, and the like.